Electrical contacts of high arc erosion resistance and method of making the same



United States Patent O 3,045,331 ELECTRICAL CONTACTS OF HIGH ARC EROSION SRESANETANCl3) AND METHOD OF MAKING THE Choh-Yi Aug, Indianapolis, and Eugene W. Burkhammer, Zionsville, Ind., assignors to P. R. Mallory & (10., Inc., Indianapolis, Ind., a corporation of Delaware No Drawing. Filed June 26, 1959, Ser. No. 824,761 5 Claims. (Cl. 29182) This invention relates to materials for electrical contact and welding electrode applications, and, more particularly, to electrical contacts having high resistance to arc erosion and'to a method of making the same.

Electrical contacts comprising a refractory constituent in the form of tungsten or molybdenum and a high conductivity constituent, such as copper or silver, have been used in the electrical arts for many years and on a very substantial scale. The principal advantage of contacts of this type is that they combine the substantial hardness and are erosion characteristics of the refractory metal with the high heat and electrical conductivity of the high conductivity metal. Heretofore, contacts of the described character have been made principally by one of two methods. 'In the first method, the powders of the pure metals have been mixed with each other and pressed into compacts. The pressed compacts have been sintered and then repressed or coined to increase the hardness and the density of the finished product. The other method comprised pressing porous compacts from the powder of the refractory metal, sintering the porous pressed refractory compacts, and subsequently infiltrating the low-melting metal of high conductivity by heat.

Considerable difiiculty has been experienced with both of the above conventional methods. The press-sinterrepress method generally resulted in insuflicient density for good physical properties and contact performance. The infiltration method was tedious, difiicult to carry out on a quantity production scale and increased manufacturing costs considerably. In most cases, it was difiicult to obtain complete absorption of the high conductivity metal in the porous body of refractory metal and, in the case of intricate shapes, it was practically impossible.

It has been discovered that the foregoing difficulties may be eliminated in a remarkably simple manner.

It is an object of the present invention to improve electrical contacts.

It is another object of the present invention to provide pressed and sintered contacts characterized by high density and high are erosion resistance.

It is also within contemplation of the invention to provide a novel and improved pressed and sintered contact composed of copper, silver, and at least one of the refractory metals molybdenum and tungsten.

The invention also contemplates a method of making electrical contacts of the described type by pressing and sintering co-reduced particles of the elementary metals.

Other and further objects and advantages of the present invention will become apparent from the following description.

Broadly stated, the invention is directed to novel and improved materials for electrical contact and welding electrode applications made by a method involving the simple pressing and sintering technique, thereby eliminating the waste and cost involved in the conventional impregnation or infiltration process. The broad range of compositions contemplated by the invention comprises at least 48 atomic percent of copper, 0.5 to 5 atomic percent of silver, and molybdenum or tungsten, or both, making up the remainder. The method used in the preparation of powder mixes and the addition of small amounts of silver make it possible to press and sinter compacts to at least 95% of the theoretical density which possess excellent physical and electrical properties.

Within the above-mentioned broad range, there are three preferred ranges, which provide excellent results. These preferred ranges are:

Copper is an excellent conductor but as a contact material it must be reinforced with molybdenum, tungsten, or both, to yield desirable properties suitable for medium to heavy duty electrical applications. It has been found that to achieve high density by a simple pressing and sintering technique, including a single pressing and sintering operation, the addition of a small amount of silver is required. Silver in solid solution in copper serves as a densifier in the sense that it changes the surface energy of the matrix, enhances wettability and prevents copper bleeding during sintering. Also, there is a slight solubility of molybdenum in silver at elevated temperatures, as high as 5.6 atomic percent molybdenum in silver-at 1600 C. The addition of silver in amounts larger than 5 atomic percent is not desirable because of the formation of a low melting copper-silver eutectic.

'In the manufacture of the contact compositions of the invention, the starting materials are suitable oxides of the constituent metals, with the possible exception of silver, which may be used in the metallic form. The refractory oxide (M00 or M00 or tungstic oxide, or both), cuprous oxide and silver powder are thoroughly blended in the desired proportions and co-reduced. The co-reduction procedure is carried out in a dry reducing atmosphere, such as cracked ammonia with a dew point of -40 F., or lower. Reduction temperature and time can be varied from 800 C. to 1100 C. and 20 to minutes to obtain complete reduction and proper Fisher average particle size (FAPS). For composition No. 2 (copper-silvertungsten), reduction is carried out in two stages, 30 minutes at 800 C. and 45 minutes at 900 C. The desirable average particle size ranges for making the three compositions listed in the foregoing are as follows:

(1) CuAg-Mo FAPS-S to 8 microns (2) CuAgW FAPS1 to 3 microns (3) Cu-AgMoW FAPS-Z to 5 microns' function of pressing pressure and powder characteristics. For the three preferred compositions, the pressed densities may vary from 45 to of the theoretical densities. 11f lubricants or binders are added to the powders to facilitate automatic pressing operations, the pressed parts should be given a presintering treatment to drive off the additives prior to final sintering. Presintering tempera- 3 tures lower than 500 C. and time at heat of about /2 hour are suflicient to serve this purpose.

The sintering operation is carried out in a dry reducing atmosphere, such as cracked ammonia with a dew point of 40 F., at temperatures-from 1200-1400 C. for a time period of /2 to 1 /2 hours. For composition No. 2, higher sintering temperatures in the range just given are used. The single most important criterion to determine the proper sintering procedure is the sintered density, which must be at least 95% of the theoretical density.

The following table summarizes the physical and electrical properties of the three preferred contact compositions of the invention. The are erosion tests under oil and in air were carried out under a load of 1400 amperes and 240 volts at a 70% power factor. It will be noted that the properties of the new materials compare very favorably'with those of a conventional copper-tungsten contact composition made by the impregnation process and also listed in the table, which is a widely used arcing contact material.

TABLE I Comparison of Properties of Compositions of the Invention and Those of Conventional Contact Material Material l 2 3 Conventional Nominal Composition in atom percent (Weight percent in parentheses) 1(0. 9) 1(1. ii 14. (16. 3) V 49(73. 3) 14. 5(31. 2) 42. 2(68) Nominal Density, g./cc. (Percent Theoretical Density in areutheses) 9. 3(99. 0) 14. 1(95. 6) 10. 5(95. 5) 14. 0(99. 5) Her ess, Rn 7378 92-98 82-90 85-92 Conductivity, percent A IAJS.i 1 6 46-47 38-40 3841 31-38 rc res on, .10,. Me. 0.1 gg; ggg 3- 3g 3- g in Air. Av. Ultimate Strength, p.s.i 53, 000 93, 000 49, 000 68, 000 Av. percent Elongation in 1" 3. 4 1.0 1. l Broke outside gage marks. Av. Modulus of Rupture, psi 112,000 153,000 62,000 120,000

It is to be observed that the present invention provides a number of important advantages. First of all, the contact compositions of the invention may be manufactured by the pressing and sintering technique, without requiring repressing or coining operations. This technique is considerably simpler and less expensive than the conventional procedures, particularly the conventional impregnation process, which was relatively expensive and resulted in considerable waste of material. In addition to eliminating the material waste involved in the impregnation technique now principally used to make conventional copper or silver infiltrated refractory (tungsten or molybdenum) contacts, the present invention also provides considerable cost saving in raw materials. This cost saving is realized [from the use of 'less expensive starting materials, the oxides. Additional savings can be obtained by using compositions embodying the invention, which are high in copper and molybdenum contents.

It is further to be noted that the method used in the preparation of powder mixes and the addition of small amounts of silver make it possible to press and sinter compacts to at least of the theoretical density which possess excellent physical and electrical properties. Due to their high resistance to arc erosion under oil and in air, these materials are most suitable for use as medium to heavy duty arcing contacts, welding electrodes, or in other applications where high electrical requirements have to be met.

Although the present invention has been disclosed in connection with a few preferred embodiments thereof, variations and modifications may be resorted to by those skilled in the art without departing from the principles of the invention. All of these variations and modifications are considered to be within the true spirit and scope of the present invention, as disclosed in the foregoing description and defined by the appended claims.

We claim:

1. A pressed and sintered electrical contact having a density at least 95% of the theoretical density composed of at least 48 atomic percent of copper, between 0.5 and 5 atomic percent of silver, and at least one metal selected from the group consisting of molybdenum and tungsten constituting the remainder, said contact being free from infiltrated constituents and from the fragility and brittleness characteristic of contacts having similar composition but containing no silver.

2. A pressed and sintered electrical contact having a density at least 95 of the theoretical density composed of :at least 48 atomic percent of copper, between 0.5 and 5 atomic percent of silver, and at least one metal selected from the group consisting of molybdenum and tungsten constituting the remainder, said contact having characteristics as are obtained by pressing and sintering co-reduced compounds of the constituent metals in the absence of infiltration and being free from the fragility and brittleness of contacts having similar composition but containing no silver.

3. A pressed and sintered electrical contact having a density at least 95% of the theoretical density composed of 68 to 72 atomic percent of copper, 0.5 to 1.5 atomic percent of silver, and molybdenum constituting the balance said contact being free from infiltrated constituents and from the fragility and brittleness characteristic of contacts having similar composition but containing no silver.

4. A pressed and sintered electrical contact having a density at least 95 of the theoretical density composed of 48 to 52 atomic percent of copper, 0.5 to 1.5 atomic percent of silver, and tungsten constituting the balance, said contact being free from infiltrated constituents and from the fragility and biittleness characteristic of contacts having similar composition but containing no silver.

5. A pressed and sintered contact having a density at least 95% of the theoretical density composed of 68 to 72 atomic percent of copper, 0.5 to 1.5 atomic percent of silver, 13 to 16 atomic percent of molybdenum, and tungsten constituting the balance, said contact being free from infiltrated constituents and from the fragility and brittleness characteristic of contacts having similar composition but containing no silver.

References Cited in the file of this patent UNITED STATES PATENTS 1,807,581 Bates June 2, 1931 2,156,802 Cooper May 2, 1939 2,162,380 Doty et al. June 13, 1939 2,227,446 Briggs et al. Jan. 8, 1941 2,283,859 Laleatos et al. May 19, 1942 2,295,334 Clark et a1 Sept. 8, 1942 

1. A PRESSED AND SINTERED ELECTRICAL CONTACT HAVING A DENSITY AT LEAST 95% OF THE THEORETICAL DENSITY COMPOSED OF AT LEAST 48 ATOMIC PERCENT OF COPPER, BETWEEN 0.5 AND 5 ATOMIC PERCENT SILVER, AND AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM AND TUNGSTEN CONSTITUTING THE REMAINDER, SAID CONTACT BEING FREE FROM INFILTRATED CONSTITUENTS AND FROM THE FRAGILITY AND BRITTLENESS CHARACTERISTIC OF CONTACTS HAVING SIMILAR COMPOSITION BUT CONTAINING NO SILVER. 